Non-volatile memory is presently designed into a large number of electronic devices that require relatively permanent storage of data even after power is removed. Common uses for non-volatile memory include personal computers, solid state drives, digital cameras, and cellular telephones. For example, program code and system data such as a basic input/output system (BIOS) are typically stored in non-volatile memory for use in personal computer systems.
Typical types of non-volatile memory include magnetic disk drives, optical disk drives, flash memory, and phase change memory (PCM). Flash memory has typically been the most common type of non-volatile memory used in electronic devices that are small and battery powered. However, flash memory is relatively slow to program that can result in a potential data bottleneck when implemented in a high speed system.
PCM is a resistive memory technology that can provide non-volatile storage but has the potential of relatively faster operation compared to flash memory. PCM, as the name implies, uses the change in resistance of a material when it changes phase in order to store data in a non-volatile manner. For example, an alloy of different elements might change from a crystalline phase having a low resistance to an amorphous phase having a high resistance. If the material could exhibit multiple distinctly different resistances, each different resistance can then be assigned a respective data value (e.g., 00, 01, 10, 11).
The phase change in PCM is brought about by heating the phase change material of each memory cell when it is addressed. This can be accomplished by a heater for each memory cell. When the heater is enabled by a current, it heats a chalcogenide alloy (e.g., germanium, antimony and tellurium (GeSbTe) or GST). When GST is heated to a relatively high temperature (e.g., over 600° C.), its chalcogenide crystallinity is lost. The GST cools into an amorphous glass-like state having a high electrical resistance. By heating the chalcogenide alloy to a temperature above its crystallization point but below the melting point it will transform back into a crystalline state having a lower electrical resistance.
FIG. 1 illustrates a schematic diagram of a portion of a typical prior art PCM array 100. The PCM array 100 includes a number of memory cells 101, each including a select device 110 coupled to a resistive storage element 111. The select devices 110 can include, for example, field effect transistors (FETs), such as MOSFETs, or bipolar junction transistors (BJTs), or diodes.
Referring to FIG. 1, the select device 110 is shown as a three terminal FET where the gate of each select device 110 is coupled to one of a number of access lines WL0-WLn (e.g., word lines). Each word line WL0-WLn is coupled in such a fashion to its respective row of memory cells. A second terminal of each FET is coupled to its respective resistive storage element 111. A third terminal of each FET is coupled to a circuit common reference. Each resistive storage element 111 is also coupled to a respective data line BL0-BLm (e.g., bit line). Each bit line BL0-BLm is coupled to its respective column of memory cells.
The word lines WL0-WLn are coupled to one or more access (e.g., row) decoders (not shown) that are used to selectively access the word lines. The bit lines BL0-BLm are coupled to the sense circuitry, through a decoder hierarchy, that senses either a voltage or a current in order to determine the programmed state of the respective memory cells that have been accessed by the word line.
One problem with PCM is the thermal transfer that can occur when an adjacent memory cell is programmed. This is typically referred to in the art as thermal disturb. The heat resulting from the programming process of one memory cell can transfer to the adjacent memory cell causing it to either be programmed or change the resistance of the already programmed cell, thus changing the valued of the stored data.
For the reasons stated above and for other reasons that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for reducing the effects of thermal disturb in a phase change memory device.